1. Field of the Invention
The present invention relates to optical disk apparatuses. More particularly, the invention relates to an optical disk apparatus for recording and/or reproducing a signal synchronously with a fine clock mark formed on an optical disk in advance as well as an optical disk apparatus capable of detecting defocus of an objective lens.
2. Description of the Background Art
A magneto-optical disk such as AS-MO (Advanced Storaged-Magneto Optics) has a fine clock mark which is preliminarily formed thereon for the purpose of obtaining a synchronization signal required for recording and reproducing a signal. The fine clock mark is generally formed by groove-shaped discontinuous regions arranged at regular intervals in a land or land-shaped discontinuous regions arranged at regular intervals in a groove.
An optical disk apparatus detects such a fine clock mark to generate a synchronization signal by a PLL (Phase Locked Loop) circuit and accordingly records/reproduces a signal on/from a magneto-optical disk in response to this synchronization signal.
If a defect such as scratch is present on the magneto-optical disk, the optical disk apparatus could erroneously detect this defect as a fine clock mark. Consequently, synchronization is lost in the PLL circuit and a correct synchronization signal cannot be obtained.
The optical disk apparatus also uses an optical pickup to direct a laser beam onto an optical disk and detect light reflected from the optical disk. The optical pickup has an objective lens for focusing the laser beam on the optical disk. The optical lens is caused to oscillate in the direction of its optical axis by means of focus servo control so as to allow the point of focus to track a signal surface of the optical disk.
If a great impact is externally applied or there is a large scratch on the optical disk, the focus of the objective lens could be deviated far from the upper face of the optical disk. Just a slight deviation may allow the focus servo control to adjust the focus back onto the optical disk. However, a greatly deviated focus cannot be adjusted back onto the optical disk. When this state is left as it is, a focus servo actuator continues moving the objective lens along its optical axis in only one of the directions. In this case, a correct focus servo control is impossible.